1,1,1,3,5,5,5-heptamethyl-3-octyltrisiloxane

Hydrolysis

Description of key information

Hydrolysis half-life: >329 h (>13.7 d) at pH 7, >9.76 h at pH 9, >5.09 h at pH 5 and 25°C (analogue read-across). The stated half-life is for removal of parent. Complete reaction to the ultimate end products will take longer.

Key value for chemical safety assessment

Half-life for hydrolysis:

13.7 d

at the temperature of:

25 °C

Additional information

No hydrolysis study is available for the submission substance, 1,1,1,3,5,5,5-heptamethyl-3-octyltrisiloxane. However, the very low solubility of this substance (2.8E-05 mg/l at 20°C, QSAR) means that the study would be technically difficult to conduct.

 

A reliable study conducted in accordance with OECD 111 is available for a structurally related substance octamethyltrisiloxane (CAS 107-51-7, EC No. 203-497-4, L3).

 

Hydrolysis half-lives of 329 h (13.7 d) at pH 7, 9.76 h at pH 9, and 5.09 h at pH 5 and 25°C were determined for L3 using a method in accordance with OECD 111 and in compliance with GLP. The results are read-across to the submission substance. The results are considered to be reliable and are used for assessment purposes.

 

Octamethyltrisiloxane (L3) a mono-constituent substance consisting of 98-99% (typical ≥98%) of L3 and <2% identified impurities (typically silane and siloxane). The submission substance is also a mono-constituent substance consisting of >97% of 1,1,1,3,5,5,5-heptamethyl-3-octyltrisiloxane and <3% identified impurities (typically dimer and tetramer). Both substances are linear siloxane chains with three silicon atoms, connected by two oxygen atoms, in which the Si-O bonds are susceptible to hydrolysis. For L3, all silicon atoms are fully substituted with methyl groups. For 1,1,1,3,5,5,5-heptamethyl-3-octyltrisiloxane, in addition to the methyl groups, the central silicon atom has an octyl side group. The octyl group in the submission substance means there is additional steric hindrance around the Si centre compared to L3. It also means that the solubility in water of the submission substances is very low, much lower than that of L3. Therefore, the reaction rate for the submission substance is expected to be slower than that of L3 and the hydrolysis half-lives for 1,1,1,3,5,5,5-heptamethyl-3-octyltrisiloxane are reported as greater than those of L3.

 

The ultimate end products of the hydrolysis reaction for L3 are dimethylsilanediol (1 mole) and trimethylsilanol (2 moles); for 1,1,1,3,5,5,5-heptamethyl-3-octyltrisiloxane, the ultimate hydrolysis products are octyl(methyl)silanediol (1 mole) and trimethylsilanol (2 moles).

 

The reaction pathway for hydrolysis of 1,1,1,3,5,5,5-heptamethyl-3-octyltrisiloxane (based on that reported for L3) is as follows. Estimates of both k1 and k2 are available for L3 based on the experimental data; the reported half-lives are based on degradation of parent substance (k1).

	k1		k2
(CH3)3SiOSi(C8H17)(CH3)OSi(CH3)3	→	(CH3)3SiOSi(C8H17)(CH3)OH + (CH3)3SiOH	→	CH3Si(C8H17)(OH)2  + (CH3)3SiOH

Regression analysis (concentration versus reaction time) was performed on the experimental data for L3 and presented in the study report. The experiments conducted at pH 7 were more susceptible to recovery decreases by partitioning of L3 into vapour phase due to the extended durations of the study. Therefore, to explicitly account for this process, the pH 7 experiments were analysed using non-linear regression. Similarly, non-linear regression was applied to the data at pH 5 and pH 9, the rate of transfer of L3 from solution to the headspace was negligible compared to rate of hydrolysis.

 

The following estimates of the rate constants for hydrolysis of parent and intermediate hydrolysis product at pH 7 and 10°C, 25°C and 35°C, obtained by non-linear regression of the measured results, were reported for L3:

 

10°C k1= 4.7E-04 h^-1,           25°C k1= 2.1E-03 h^-1,           35°C k1= 4.9E-03 h^-1

10°C k2= 3.9E-04 h^-1,           25°C k2= 1.9E-03 h^-1,           35°C k2= 5.3E-03 h^-1

 

The rate of reaction of the intermediate hydrolysis product was similar to that of the parent substance.

 

Hydronium and hydroxide catalysed rate constants were reported for L3. The estimated contributions from hydronium and hydroxide ion catalysis were found to account very well for the observed rates of hydrolysis at neutral pH.

 

The hydronium and hydroxide catalysed rate constants may be used to calculate the hydrolysis rate at any pH according to the equation:

k_obs= k_H3O+[H3O⁺] + k_OH-[OH^-]

 

The calculated half-life of the substance at pH 4 is 0.5 h and that at pH 2 is 0.005 hours (18 seconds).

 

Reaction rate increases with temperature therefore hydrolysis will be faster at physiologically relevant temperatures compared to standard laboratory conditions. For the submission substance, the hydrolysis half-life at 37.5ºC and pH 7 (relevant for lungs and blood) is calculated as >115 hours. At 37.5ºC and pH 2 (relevant for conditions in the stomach following oral exposure), the hydrolysis half- life is calculated as >9 seconds. At 37.5°C and pH 5.5 (relevant for dermal exposure), the hydrolysis half-life is calculated as >7 hours.

 

In a further study report (Dow Corning Corporation 2016), hydrolysis half-lives over a range of pH at 25°C, 12°C and 9°C were calculated using data from the key study for L3 (Dow Corning Corporation 2007). 12°C and 9°C are considered broadly representative of freshwater and marine environments, respectively. The Arrhenius parameters were derived by linear regression analysis of the original experimental data, and subsequently rate constants were calculated for temperatures and pH range of interest.

	Half-life at 9°C (days)	Half-life at 12°C (days)	Half-life at 25°C (days)
pH 6	7	5.5	2.1
pH 7	58	44.1	13.5
pH 8	31.9	21	3.9

For the environmental exposure assessment, the parent substance will be considered because the half-life for hydrolysis of the parent is greater than 12 hours at pH 7.

 

Hydrolysis of the read-across substance decamethyltetrasiloxane (CAS 141-62-8)

Data for the substance decamethyltetrasiloxane (CAS 141-62-8, L4) are read-across to the submission substance 1,1,1,3,5,5,5-heptamethyl-3-octyltrisiloxane for appropriate endpoints (see Section 1.4 of the CSR). The hydrolysis half-lives of the two substances is relevant to this read-across, as discussed in the appropriate Sections of the CSR for each endpoint.

 

For decamethyltetrasiloxane, hydrolysis half-lives at 25°C of 14 h at pH 5, 728 h (30.3 days) at pH 7 and 21.1 h at pH 9 were determined in accordance with OECD 111 (Dow Corning Corporation, 2009). The predicted result is supported by hydrolysis half-lives of 3.6 h at pH 4, 630 h at pH 7 and 5.3 h at pH 9 and 20 -25°C.

 

The ultimate products of hydrolysis are dimethylsilanediol and trimethylsilanol.

 

Hydrolysis of the read-across substance dodecamethylpentasiloxane (CAS 141-63 -9)

Data for the substance dodecamethylpentasiloxane (CAS 141-63-9, L5) are read-across to the submission substance 1,1,1,3,5,5,5-heptamethyl-3-octyltrisiloxane for appropriate endpoints (see Section 1.4 of the CSR). The hydrolysis half-lives of the two substances is relevant to this read-across, as discussed in the appropriate Sections of the CSR for each endpoint.

 

The hydrolysis half-life of L5 (CAS 141 -63 -9) has been read-across from the structurally related substance L4 (CAS 141-62-8). Hydrolysis half-lives at 25°C of 30.3 d (728 h) at pH 7, 14 h at pH 5, and 21.1 h at pH 9 were determined at room temperature for L4 in accordance with OECD 111 (Dow Corning Corporation 2009).

 

L4 is a linear siloxane chain with four silicon atoms, connected by three oxygen atoms, in which the Si-O bonds are susceptible to hydrolysis. All silicon atoms present are fully substituted with methyl groups. L5 is a structurally related linear siloxane, with five silicon atoms and four oxygen atoms.

 

As well as being structural analogues, both siloxanes have consistent physicochemical properties including high molecular weight (310 and 384 g/mol respectively), very high log K_ow (above 8 for both substances) and very low solubility in water (7E-03 mg/l for L4 and 7E-05 mg/l for L5). The substances generally possess similar physicochemical properties. There are no significant steric differences between the Si centres in the two structures. Therefore, the rate of reaction at pH 7 is expected to be approximately the same. The ultimate end products of the hydrolytic reaction, dimethylsilanediol and trimethylsilanol, will be the same for both structures. The stated half-life is for removal of the registration substance due to hydrolysis.

 

Half-life values of approximately 6.6 h at pH 4, 2000 h at pH 7 and 14 h at pH and 20-25°C were obtained for L5 using a validated QSAR estimation method.